In 1828, Friedrich Wöhler, a German physician and chemist by training, published a paper that describes the formation of urea, known since 1773 to be a major component of mammalian urine, by combining cyanic acid and ammonium in vitro. In these experiments the synthesis of an organic compound from two inorganic molecules was achieved for the first time. These results weakened significantly the vitalistic hypothesis on the functioning of living cells, although Wöhler, at that time, was more interested in the chemical consequences of isomerism than in the philosophical implications of his finding. However, the chemical synthesis observed by Wöhler does not represent the reaction which is employed in the mammalian liver for urea synthesis. The mechanism of this process was elucidated by the German physician Hans A. Krebs and his medical student Kurt Henseleit in 1932 and was shown to include the ornithine cycle. This 'urea cycle' is only observed in living cells; this apparently vitalistic phenomenon is caused by the compartmentalization of the various enzymatic reactions in mitochondria and cytosol, respectively.